1. Field of the Invention
The present invention relates to a television circuit, and more particularly, to a cathode ray tube (CRT) protection system for protecting a CRT from any residual high voltage remaining after power is turned on/off or from abnormal operation in a projection-type television.
2. Description of the Related Art
A projection-type cathode ray tube (CRT) used in a large screen display projects strong current beams onto a small area. This causes residual high voltage to be maintained when power is turned on/off or when a CRT driver circuit is broken in an abnormal state. If the CRT is broken in a projection display, the broken CRT must be replaced and is costly.
FIG. 1 is a block diagram showing a configuration of a conventional cathode ray tube device. FIG. 1 relates to a high voltage variation control circuit using a variable pulse width modulation (PWM) pulse. When beam variations occur in a CRT 150, a flyback transformer 110 provides a beam variation signal to a PWM processor 120. The PWM processor 120 detects an error value from the beam variation signal and a reference signal, and differentially amplifies the detected error value. Subsequently, the PWM processor 120 compares a horizontally blanked sawtooth wave with the detected error value and provides a pulse waveform according to the detected error value. A high voltage output unit 100 compensates for a reference high voltage pulse with the pulse waveform in accordance with the detected error value to provide an output to the CRT 150. A signal processor 130 processes video and audio signals for display on the CRT 150 and provides the processed video and audio signals to the CRT 150 through a CRT driver 140.
According to conventional technology, when the power supply of a television is operated, it happens that a charged, residual high voltage is maintained in a condenser of the CRT during a state in which deflection is not performed. This causes a spot phenomenon wherein a strong residual beam is focused onto a point of the CRT when the television power is turned off, resulting in a breakdown of the CRT.
Furthermore, in an abnormal state (for example, internal discharge frequently occurring in a beam CRT), driving parts of the CRT driver malfunction and an excessive medium voltage power supply flows to ground. The medium voltage power supply blows out a CRT protection resistor and cuts off the CRT driver circuit. This generates a maximum beam in the CRT adopting a reverse polarity drive mode and drives a screen into a state in which auto brightness limit (ABL) control is not possible. In this situation, if the power supply is cut off, a strong spot occurs, causing the CRT to malfunction.
It is an object of the present invention to provide a cathode ray tube (CRT) protection system for detecting that a power supply is quickly turned off from a system power supply, to enable the cutting off of a high voltage in order to protect the CRT.
It is another object of the present invention to provide a CRT protection system for detecting a medium voltage power supply occurring when the CRT operates in an abnormal state, to enable the cutting off of a high voltage in order to protect the CRT.
It is still another object of the present invention to provide a CRT protection system for protecting a CRT from maintained, residual high voltage, occurring when power is turned on/off or due to abnormal operation.
To achieve the first object of the present invention, there is provided a CRT protection system, in a projection television, comprising: a high voltage output unit for providing a high voltage used for driving a CRT; a high voltage variation detector for detecting a varied high voltage of the high voltage output unit; a CRT driver for providing a predetermined voltage for driving the CRT; a medium voltage power supply fuse short-circuit detector for detecting a short-circuit state of a medium voltage fuse based on the operation state of the CRT driver; and a pulse width modulation processor for generating a pulse width modulation signal for limiting output of the high voltage output unit depending on outputs of the high voltage variation detector and the medium voltage power supply fuse short-circuit detector when the medium voltage fuse short-circuits, regardless of the varied high voltage.
To achieve the second object of the present invention, there is provided a CRT protection system, in a projection television, comprising: a high voltage output unit for providing a high voltage used for driving a CRT; a high voltage variation detector for detecting a varied high voltage of the high voltage output unit; a falling time power supply detector for detecting a power supply having the shortest falling time from power supplies driving the projection television; a power supply off time detector for providing a voltage signal indicating the on/off state of the power supply detected by the falling time power supply detector; and a pulse width modulation processor adopting a pulse modulation mode controllable for limiting the output of the high voltage output unit based on the outputs of the high voltage variation detector and the power supply off time detector when the detected power supply is in the off state.
To achieve the third object of the present invention, there is provided a CRT protection system, in a projection television, comprising: a high voltage output unit for providing a high voltage used for driving a CRT; a high voltage variation detector for detecting a varied high voltage of the high voltage output unit; a CRT driver for providing a predetermined voltage for driving the CRT; a medium voltage power supply fuse short-circuit detector for detecting a short-circuit state of a medium voltage fuse based on the operation state of the CRT driver; a falling time power supply detector for detecting a power supply having the shortest falling time from power supplies driving the projection television; a power supply off time detector for providing a voltage signal indicating the on/off state of the power supply detected by the falling time power supply detector; and a pulse width modulation processor for receiving the varied high voltage from the high voltage variation detector, the short-circuit state from the medium voltage power supply fuse short-circuit detector, and the power on/off voltage signal from the power supply off time detector and generating a pulse width modulation signal for controlling the output of the high voltage output unit when the medium voltage fuse is in a short-circuited state or when the detected power supply is in the off state, regardless of the varied high voltage.